1. Field of the Invention
The invention relates to electric heating irons used for melting hot-melt seaming tape during installation of wall-to-wall carpeting, and more particularly to the method of temperature control for such seaming irons.
2. Description of the Related Art
During the installation of wall-to-wall carpeting, often the area to be carpeted is wider than the carpet as produced by the carpet mill. Therefore, two or more pieces of carpet must be joined along a seam. Wall-to-wall carpet is usually seamed using hot-melt carpet seaming tape. Carpet seaming tapes typically include a layer of thermoplastic (hot-melt) resin adhesive for bonding to the adjoining pieces of carpet. To form a seam using the face-seaming process, the two pieces of carpet to be seamed are placed with the tufts facing up and edges abutting over a length of seaming tape. The seaming tape is centered under the two abutting edges with the layer of adhesive facing the carpet backing. A hot carpet seaming iron, or other suitable implement, is used to melt the thermoplastic adhesive and the carpet backing is pushed into the molten thermoplastic adhesive. The adhesive quickly sets, forming a permanent seam. This method is disclosed by, e.g., U.S. Pat. Nos. 3,972,768, 4,097,445 and 4,536,244.
Various carpet seaming irons are commercially available from several sources. All employ similar design elements: a relatively thin, thermally insulated heating platen, an electrically powered heating element, a handle, and an electric circuit incorporating a thermostat for temperature control. Such irons are disclosed by, e.g., U.S. Pat. No. 4,536,244. The iron platen must be thin to allow the platen to fit readily underneath the two piece of carpeting. The electric power must also be relatively high (typically, irons consume about 800 Watts of power) so that the iron heats up quickly and maintains its temperature. Otherwise, the iron will not melt the carpet seaming tape quickly enough for the typical operator.
The combination of a thin platen, high power, and thermostatic control has several disadvantages. Thermostats are prone to react too slowly to temperature changes. A high-powered, low mass iron heats up so quickly that the thermostat will not shut off the power to the heater in time. That is, the iron "overshoots" the desired temperature. The platen will become hotter than desired, which can damage the carpet or seaming tape, and shorten the life of the iron. Excessive smoke can be produced when the seaming tape adhesive is overheated. After the thermostat shuts off power, the thin platen cools rapidly. Again, the thermostat reacts too slowly. The iron becomes too cool for the operator to work rapidly. The thermostat then activates the electric heater, and the iron begins heating again, repeating the cycle. The iron cycles repeatedly between a temperature that is too hot and a temperature that is too cool. The thermostat is unable to maintain a steady iron temperature.
A further disadvantage of thermostatic control is that the full power of the heater is switched through the contacts of the thermostat. After many cycles, the contacts can fuse in a permanently closed position. The iron heater will remain permanently on and can overheat dangerously.
A still further disadvantage of thermostatic control is that many commonly available thermostats do not consistently control within the same range of temperature. For example, a given thermostat may control in a range between 350.degree. F. and 420.degree. F. Another thermostat from the same manufacturing lot may control in a range between 370.degree. F. and 440.degree. F. Consequently, the seaming irons do not perform consistently. One may run relatively cool, while another runs relatively hot.